You’re strong when you run flats but as soon as you hit a hill, you’re gasping like a fish on land. Here’s what’s going on.

If air pollution is the cause of your wheezing, try running earlier in the morning.
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Running on an incline will ramp up the intensity of your workout—so it makes sense that you feel fatigued and out of breath while you're racing (or slogging) to the top. "You'll have to work harder than usual, and you may be huffing and puffing," says Albert Rizzo, MD, senior medical advisor for the American Lung Association. But if you're actually experiencing chest tightness or noisy, obstructed breathing, there may be a more serious issue at play.

First, take note of when exactly you experience these wheezing episodes. "Ask yourself if it happen every time you run up any hill, anywhere, or does it happen in one certain geographic location, during certain weather, or during a certain time of day," says Rizzo.

Exercise-induced breathing problems are often affected by environment and air quality: Cold, dry air can inflame airways (especially for mouth-breathers), while the air on very hot days often has elevated levels of pollutants. (Harmful ozone gasses are most easily formed on humid, sunny days when air is stagnant.)

Your best option, in this case, is to limit your exposure to poor air quality by exercising early in the morning or away from direct sources of pollution (like running along a main thoroughfare). If you must run outdoors on bad days, skip the hills and keep your effort level at easy.

If pollution or cold, dry air don't seem to be issues—like if your chest feels like it's whistling or rattling repeatedly, even in clean and moderately temperate conditions—you could have exercise-induced asthma. "It's worth getting checked out and diagnosed by a physician, because it's a very treatable disease," says Rizzo.

Your doctor will confirm whether or not you’re healthy enough to undertake strenuous activity, and can provide advice on how to breathe easier while doing it.

One of these is to make sure you complete an extended warm up before hitting the hills. Research has shown that a gradual increase in exercise intensity can desensitize lungs for up to 80 minutes from the stress of heavy breathing, and a 2012 review of previous studies found that warm-ups that include at least some high-intensity intervals provided the most protection—think 10 minutes of easy jogging followed by 5 minutes of alternating speed drills and recovery periods.

Once diagnosed with asthma, you may also be prescribed a "rescue" inhaler that can be used preventatively, 20 to 30 minutes before doing an activity you know will trigger wheezing. Losing excess weight and staying hydrated may also help reduce airway inflammation.

No matter what the circumstances, it's worth a mention to your doctor to rule out a serious heart or lung condition. "Healthy athletes shouldn't experience wheezing during a workout—not even a hard one," says Rizzo. "Your physician can help you get to the bottom of it and, hopefully, get you some relief."